Resistor assembly

ABSTRACT

A resistor assembly capable of withstanding mechanical stresses and high frequency as well as d.c. electrical surges greater than the aggregate voltage rating of its component resistor elements. A plurality of resistor elements are divided into sections, each section bridged by a spark gap. The spark gaps are series connected in a linear sequence between the terminals of the assembly and electrically connected to their respective resistor elements by leads positioned orthogonal to the line of spark gaps. The resulting configuration has a ratio of impedance along the spark gaps to impedance along the resistor elements which is high for steady state low frequency voltage applications and low for large, high-frequency voltage surges. The resistor elements are encapsulated in a thermally conductive, electrically insulating block of epoxy, and advantageously employed as a voltage divider in high voltage charged particle accelerators.

United States Patent [191 Charpentier [451 Sept. 4, 1973 RESISTORASSEMBLY Robert R. Charpentier, Chelmsford, Mass.

[73] Assignee: High Voltage Engineering Corporation, Burlington, Mass.

22 Filed: June 2, 1972 21 Appl. No.: 259,234

[75] Inventor:

Primary Examiner-Roy Lake Assistant ExaminerDarwin R. HostetterAtt0rneyRussell & Nields [57] ABSTRACT A resistor assembly capable ofwithstanding mechanical stresses and high frequency as well as do.electrical surges greater than the aggregate voltage rating of itscomponent resistor elements. A plurality of resistor elements aredivided into sections, each section bridged by a spark gap. The sparkgaps are series connected in a linear sequence between the terminals ofthe assembly and electrically connected to their respective resistorelements by leads positioned orthogonal to the line of spark gaps. Theresulting configuration has a ratio of impedance along the spark gaps toimpedance along the resistor elements which is high for steady state lowfrequency voltage applications and low for large, highfrequency voltagesurges. The resistor elements are encapsulated in a thermallyconductive, electrically insulating block of epoxy, and advantageouslyemployed as a voltage divider in high voltage charged particleaccelerators.

5 Claims, 7 Drawing Figures RESISTOR ASSEMBLY BACKGROUND OF THEINVENTION 1. Field of the Invention This invention pertains toelectrical resistors, and more particularly to resistor assemblieshaving selfprotecting means to prevent damage from voltage surges.

2. Description of the Prior Art Resistors capable of supporting high DC.voltage levels are called for in applications such as voltage dividersand calibration elements associated with the voltage source andacceleration sections of charged particle accelerators. When so used theresistors are subjected to tube sparking and other short durationvoltage surges encountered in accelerators. The voltage surges typicallylast for a period of several microseconds and may exceed the normal DC.voltage across the resistor by a factor of 2 or more, causingconsiderable problems of resistor breakdown and instability in ohmagevalue. In addition, resistors used as voltage dividers in accelerationtubes operate in an environment of high pressure insulating gas and mustbe able to withstand several megarads of ionizing radiation. They arealso subject to serious mechanical stresses from handling and heatingduring operation, and must be kept to a size small enough to fit in withthe other accelerator elements.

Several excellent types of resistors are available commercially, butnone have been found to perform with a satisfactory degree ofreliability under the harsh conditions described. High voltage carboncompositions have been used, but are liable to a considerable reductionin resistance with increased voltage. Wire-wound resistors are bulky andexpensive and tend to breakdown under high voltage surge conditions.Metal film resistors have problems similar to wire wound and are evenmore costly. Some success has been achieved with specially compoundedcarbon inks applied in a spiral on a steatite substrate, but surgingconditions have lim- 40 ited their use. Metal oxide films, deposited onsteatite substrates, are characterized by a low voltage coefficient, butonce again breakdown, have a sputtering tendency during surges.

Problems other than overvoltage breakdown have also been experienced inthe accelerator field. Resort has been made to encapsulating single highvoltage resistors in epoxy resins to prevent deterioration of theresistor due to corona sputtering and deleterious effects of the highpressure insulating gas getting inside the resistor. While alleviatingthese problems, the encapsulation process has itself led to otherdifficulties, such as the creation of harmful mechanical stresses in theresistor from epoxy shrinkage during preparation of the unit. The epoxymust also have a thermal conductivity sufficiently high to dissipateadequately IR losses from the resistor.

SUMMARY OF THE INVENTION The present invention contemplates a resistorassembly with a self-protecting surge voltage bypass mechanism thateffectively solves the above problems encountered in the prior art, andwhich can be conveniently adapted to the particular voltage requirementsof various high voltage applications. It is most applicable underconditions of low frequency or DC. steady state voltages subject totransients and high frequency voltage surges, such as encountered inparticle accelerator conditioning periods. The assembly is divided intoone or more sections, each section consisting of a resistor elementbridged by a spark gap. The various sections are series connectedbetween spark gap poles in a configuration whereby the successive sparkgaps present a lower impedance for high frequency voltage surges thanfor low frequency or DC. steady state conditions, while the path alongthe resistor elements has a lower impedance for low frequency or DC.steady stae conditions than for high frequency voltage surges.

Such voltage surges tend to propagate in a straight line, resistingchanges in the direction of propagation. For example, accelerator tubesparks, having a time constant typically in the order of nanoseconds,have been found to exhibit a straight line tendency. This phenomenon hasbeen turned to advantage in the present invention by the use of sparkgaps arranged in a straight line between the terminals of the resistoras sembly, and by connecting the resistor element of each station toeach pole of its associated spark gap by connections orthogonal to theline of spark gaps. Damaging voltage surges are thereby constrained tothe spark gap path. The operating low frequency, or DC voltage, ismaintained across the series connected resistor elements, as well astheir protective gaps.

In the preferred embodiment the resistor elements each comprise twoseparate units, each unit having a ceramic core with a spiral line ofresistive metal oxide material deposited thereon. The resistor unitsdepend from opposite poles of the bridging spark gap at right angles tothe line of spark gaps and are electrically connected to each other atthe ends away from the spark gaps by metallic strips.

The resistor elements are encapsulated in a single specially preparedblock of insulating epoxy resin to reduce electric field stresses at theedges of the resistor elements and metallic connectors and to shield theresistor elements from deterioration due to immersion in an ambient highpressure insulating gas medium. Resin shrinkage during casting andconsequent mechanical stresses are limited by mixing a high level offiller material with the resin. In the preferred embodiment the totalresistance of the assembly can be varied during manufacture while thesame overall dimensions are retained by the use of a single castingmold. The addition or elimination of an appropriate number ofresistorspark gap sections may be effected for wattage requirements.Voids are filled in by the epoxy mixture, and leads are provided toconnect electrically the end sections to the assembly terminals. Anintimate contact between the resistor elements and case insulation isachieved that aids in heat dissipation and protects the resistorelements from thermomechanical shock.

The resistor assemblies are highly useful in maintaining a voltagegradient between equipotential planes of a particle accelerator. When soemployed they are in close proximity to one another and can be providedwith conductive collars between the terminals and outer spark gaps,whereby a small gap is left between the collars of adjacent resistorassemblies. Voltage surges entering an assembly are shunted from collarto collar due to skin effect phenomena and bypass the resistor elementsand spark gaps entirely.

The resistor assembly described is mechanically durable and can receiverepeated surges of more than 10 times the voltage rating of theindividual resistor units without damage. Additional advantages willbecome apparent from the ensuing detailed description.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a view in cross-section showingthe resistor assembly embodying the invention.

FIG. 2 is a side view in cross-section taken along the line 2-2 of FIG.11 of one of the resistor units of FIG. 1.

FIGS. 3 and 4 are cross-sectional views showing variations in theconfiguration of resistor elements.

FIG. 5 is an overhead broken view showing a resistor assembly spanningtwo equipotential planes in a charged particle accelerator. One-half ofthe upper equipotential plane is shown in the top portion of the figure,and one-half of the lower equipotential plane in the bottom portion ofthe figure.

FIG. 6 is an overhead view in partial cross-section showing means forattaching a resistor assembly in a charged particle accelerator.

FIG. 7 is a view in cross-section showing a compact resistor assemblywith a single spark-gap and collars near the terminals.

DETAILED DESCRIPTION OF THE INVENTION In a preferred embodiment of theinvention, a resistor assembly is divided into a number of similarsections, one of which is shown enclosed in a dotted line in FIG. l andgenerally indicated by the numeral 2. Except for modification at eachend of the assembly, the various sections are identical and adescription of the selected section 2 is equally applicable to theothers.

The section 2 has as its major components a resistor element and a sparkgap element 3. The resistor element consists of two resistor units 4,which may be ccramic core structures with a spiral line of resistivemetallic oxide material deposited along its surface, and which areelectrically joined at one end by a metallic link 6 screwed ontoresistor caps 8. A spark gap 3 is formed from a pair of longitudinallyaligned spaced metallic bars 10, the adjacent ends of which serve asspark gap poles 12. Each resistor unit 41 is electrically connected to aspark gap pole 12 by means of a conductive resistor cap M, lead I6oriented at a right angle to the metallic bars 10, and fastening screwK8 in the metallic bar 10.

Additional sections are provided on either side of section 2, connectedbetween spark gaps to form a continuous line of spark gaps, each havingan, associated resistor element. As shown in the drawings this canconveniently be accomplished by utilizing as poles for additional sparkgaps 20 the ends of metallic bars 10 not employed as poles 12 for sparkgap 3. Complementary poles for spark gaps 20 are provided by endportions of metallic bars 22, the other ends of which in i turn functionas poles for further spark gaps. A preferred method of accomplishingsuch an arrangement employs a plurality of repeated resistivestructures, each structure consisting of a metallic bar W and twodepending resistor units ll. The structures are constructed separatelyand then joined by links 6 in forming the assembly.

All the resistor units 4 are encapsulated within a single insulatingblock 24 of thermosetting epoxy resin having a filler content of atleast 80 percent. Although such a large proportion of filler, which ispreferably tabular alumina, causes some reduction in the electricalinsulating capacity of the resin, it has been found to reduce shrinkageduring molding to less than one-half percent and consequently to avoidthe imposition of harmful mechanical stresses on individual resistorunits 4. In practice the amount by which the resins insulating quantityis decreased is not enough to limit the use of the resistor assembly inany accelerator applications.

The action of a high frequency voltage surge is best described withreference to FIG. ll. During steady state D.C. conditions a voltage isestablished between metallic terminals 26 and 28. Current flows from oneend to the other through the resistor units A, metallic links 6 andmetallic bars, setting up across each spark gap a proportional amount ofthe total voltage difference. When a high frequency voltage surge entersone of the terminals, however, the tendency of the surge to propagate ina straight line causes it to bypass the repeated right angle turns, 30and 32 preceeding the first resistor unit 34 and to jump across thefirst spark gap 36. Thereafter the orthogonal relationship between theleads l6 and the metallic bars which form the spark gaps force the surgeto travel from spark gap to spark gap by flashover across the gaps,rather than along the lower steady state resistance path offered by theresistor units 4 The surge will continue to break down spark gaps untilit either exits at the opposite terminal or is dissipated.

The dimensions and electrical characteristics of the various elementscan be selected according to the respective magnitudes of the D.C.operating voltage and expected voltage surges. One assembly developed byapplicant, incorporating 40 resistor units and 21 spark gaps, had aseparation of 0.015 inch between spark gap poles, the resistor unitsbeing about one-fourth inch in diameter and 1% inch long. Whileoperating under a normal voltage requirement of 50 kilovolts, it wassubjected to and withstood without damage or change in resistancecharacteristics thousands of surges of over one megavolt and onemilliamps each lasting for several microseconds.

It is an advantage of the invention that the resistance of theassemblies can be conveniently modified without changing the over-allassembly dimensions by varying the number of resistor units in the samemold. An assembly having a reduced number of resistor units and sparkgaps is shown in FIG. 3. The space previously occupied by resistor unitsat each end 38 of the assembly is filled in with epoxy resin. Conductingrods 40 connect the terminals 26 to conducting studs 42, which studs areelectrically connected to interior resistor units 4 through two rightangle turns. Extended metallic bars 44, providing the outside pole ofthe first spark gaps, are exterior to the encapsulating resin block andelectrically join the terminals 26 to conducting studs 42. The assemblycan be further varied by adding or eliminating other resistor sections.

Although the ladder-like resistor arrangement thus far described permitsa relatively large number of resistor units to be used, otherarrangements employing the principles of the invention might also bedesired. In FIG. 4, for instance, resistor units d6 are longitudinallyaligned, with each end connected to leads 48 that are orthogonallyjoined to an exterior metallic spark gap bar 50. Each section of theassembly has only one longitudinal resistor unit 66, permitting areduction in assembly size.

Referring now to FIGS. 5 and 6, a resistor assembly 52 may be placedacross adjacent equipotential planes in a charged particle acceleratorby connecting opposite ends of the assembly to adjacent equipotentialrings 54 and 56. A preferred mode of placement, in an accelerator havinga tube 58 and mounting pads 60 located inside equipotential rings 54 and56, utilizes metallic supports 62 and 64 clamped to adjacent rings 54and 56 respectively. A rounded plug 66 on one terminal 68 of theresistor assembly 52 lodges against and partially into a socket 70 insupport 62. At the other terminal 72 a plug 74 extending from support 64fits in an opening 76 in the terminal and compresses a spring 78provided therein to facilitate attachment and removal of the resistorassembly 52. To prevent rotation of the assembly 52 a peg 79 on support64 fits into another opening 80 in terminal 72. An adequate electricalconnection is assured at terminal 68 by bolting thereon a lead 82 fromequipotential ring 54.

For low power operations requiring a resistor assembly of smallerdimensions, an embodiment shown in FIG. 7 having a plurality of seriesconnected resistor units encapsulated in epoxy 84 may be used. Only onespark gap 86 is provided, the poles 88 of which are connected to the endresistor units 90 by metallic clips 92. These clips 92 are shaped torequire a surge to turn a right angle before entering the resistorunits. Additional protection may be afforded by attaching conductingtoroidal collars 94 at the assembly terminals. The collar diameters aresuch that when a plurality of resistor assemblies are emplaced in aparticle accelerator a gap of approximately 0.1 inch is left between thecorresponding collars of any two adjacent assemblies. With such aspacing electrical surges tend to flash over between the resistorassemblies along the collars 94 instead of entering the assemblies. Thespark gaps 86 provide a second line of protection against any surgesthat might get in.

Having now described the invention and several embodiments thereof, itmay occur to one skilled in the art to adopt particular variations ormodifications for certain purposes. It is my intention therefore thatthe embodiments shown herein are for purposes of illustration, and theinvention is to be limited only in terms of the appended claims.

I claim:

1. In a charged particle accelerator having an acceleration tube and aplurality of equipotential rings for grading an applied voltage alongthe length of the tube, a surge protected resistor assembly formaintaining the voltage between two adjacent rings, said resistorassembly comprising:

a. a pair of terminals adapted to be held in electrical communicationbetween said adjacent equipotential rings,

b. one or more series connected spark gaps, said spark gaps disposed insubstantially a straight line between said terminals, the outer pole ofone of the outermost spark gaps electrically connected to one of saidterminals and the outer pole of the other outermost spark gapelectrically connected to the other of said terminals,

. each of said spark gaps having a resistor element associated therewithbridging its poles, each said resistor element electrically connected tothe poles of its associated spark gap by conducting means stemming fromsaid spark gap at substantially a right angle to said line of sparkgaps.

2. The charged particle accelerator of claim 1 wherein said resistorelements are encapsulated in a block of solid electrically insulativematerial.

3. The charged particle accelerator of claim 2 wherein said resistorassemblies are immersed in an insulating gas having a dielectricconstant less than that of said insulative material.

4. The charged particle accelerator of claim 1 wherein the terminals ofsaid resistor assemblies are provided with conducting toriodal collarsof sufficient size to leave a small gap between the correspondingcollars of adjacent assemblies.

5. The charged particle accelerator of claim 3 wherein said gap betweentoroidal collars is approximately 0.10 inch.

1. In a charged particle accelerator having an acceleration tube and aplurality of equipotential rings for grading an applied voltage alongthe length of the tube, a surge protected resistor assembly formaintaining the voltage between two adjacent rings, said resistorassembly comprising: a. a pair of terminals adapted to be held inelectrical communication between said adjacent equipotential rings, b.one or more series connected spark gaps, said spark gaps disposed insubstantially a straight line between said terminals, the outer pole ofone of the outermost spark gaps electrically connected to one of saidterminals and the outer pole of the other outermost spark gapelectrically connected to the other of said terminals, c. each of saidspark gaps having a resistor element associated therewith bridging itspoles, each said resistor element electrically connected to the poles ofits associated spark gap by conducting means stemming from said sparkgap at substantially a right angle to said line of spark gaps.
 2. Thecharged particle accelerator of claim 1 wherein said resistor elementsare encapsulated in a block of solid electrically insulative material.3. The charged particle accelerator of claim 2 wherein said resistorassemblies are immersed in an insulating gas having a dielectricconstant less than that of said insulative material.
 4. The chargedparticle accelerator of claim 1 wherein the terminals of said resistorassemblies are provided with conducting toriodal collars of sufficientsize to leave a small gap between the corresponding collars of adjacentassemblies.
 5. The charged particle accelerator of claim 3 wherein saidgap between toroidal collars is approximately 0.10 inch.